Many attempts have been made in the prior art to improve the frequency response characteristics of horn-type loudspeakers. Typically, a horn-type loudspeaker includes a driver connected to an initial throat section of the horn, with the throat section opening into either a middle or frontal section of the horn. One example is shown in British Pat. No. 247,532, completed for acceptance in July 5, 1927, where a horn-type sound amplifier is disclosed having a plurality of metal plates mounted on vibration insulators within a frontal portion of the horn, so that the plates are free to vibrate in response to high frequency vibrations, for producing an effective amplification of the sound at those frequencies. Another example is shown in British patent application No. GB200668A, filed on June 27, 1978, and taking priority from U.S. application Ser. No. 810,642, filed June 27, 1977 now ABN, which discloses a horn-type loudspeaker system having a substantially elongated throat section for improving the uniformity of the frequency response characteristic of the loudspeaker.
A loudspeaker horn taught in British patent specification No. 495,594, accepted on Nov. 16, 1938, discloses the use of vertical partition walls within the frontmost portion of the loudspeaker horn for subdividing this portion into horn sections that vary exponentially in cross-section, for improving the frequency response of the horn. In this example, the throat section of the horn is small in comparison to the wavelengths of the frequencies that are to be projected via the horn.
In U.S. Pat. No. 2,537,141, issued on Jan. 9, 1951, a loudspeaker horn is disclosed having vertical deflectors located within a middle section of the horn, and curved horizontal baffles within a frontal and forwardmost section of the horn, for both improving the middle and high audio frequency response, and increasing the angle of radiation of sound from the horn area. The deflectors in the middle section of the horn cause sound to diffuse in one plane, whereas the deflectors in the forwardmost section of the horn cause sound to diffuse in a plane that is perpendicular to the plane of diffusion of the middle or center section of the horn. The exit mouth of the horn is semicircular or terminates in an arc.
U.S. Pat. No. 3,852,529, issued Dec. 3, 1974, teaches the use in an acoustic horn of spaced longitudinal ribs extending into the horn for reducing the cross-sectional area of its throat for minimizing phase cancellation between transmitted acoustic wavefronts, and for providing a broadband impedance transformation. The ribs extend from the mouth of the loudspeaker horn into the throat of the horn near the driver connected to the back of the throat. Also, illustrated in FIG. 2 of this patent is what is described as a prior art horn having a "phase correction plug therein to reduce the phase cancellation problem." The phase correction plug is shown to have a plurality of passages of substantially constant path length between a diaphragm of a transducer connected at the back end of the throat and the entry into the major portion of the horn, for correcting phase cancellation at high frequencies. The use of such a plug is further described as a technique that "provides some phase cancellation correction, the correction is not complete because the length of the pasages is not identical, and the plug may have an adverse effect on other performance characteristics of the horn. Furthermore, the plug must be made to close tolerance specifications and the distance between the cone and the plug is critical."
In U.S. Pat. No. 4,091,891, issued on May 30, 1978, a horn speaker is disclosed having within the sound passage of the horn two partition walls extending from the throat to the mouth of the horn, and arranged symmetrically with respect to the principal axis to the horn for dividing the horn into three sound passages. The outer two sound passages are curved and greater in length than the central most sound passage about the principal axis of the horn, for both increasing the width of the angle of sound distribution from the horn and for improving the frequency response characteristics of the horn.
In U.S. Pat. No. 4,325,456, issued Apr. 20, 1982, an acoustical transformer or a phasing plug is disclosed for coupling the sound from an annular diaphragm to the throat of a compression-type loudspeaker, for improving the impedance match between the output of a driver or annular diaphragm connected to the input or entry port of the horn. The phasing lug is formed from a cone having spaced radially slots or channels formed therein connecting the truncated surfaces of the cone, for forming air passageways for propagation of sound waves, so that the channels are tapered such that the cross-sectional areas of the channels increase from their inlet ends near the speaker diaphragm, towards their outlet ends positioned at the throat of the horn.
Another example is shown in U.S. Pat. No. 4,390,078, issued June 28, 1983, where a loudspeaker horn is disclosed including a central vane located in a median section of the horn for maintaining an exponential flare rate and dividing the median section into two vertical slits, for providing a substantially constant coverage angle and wide frequency range of operation for the horn.
Although these prior art examples of horn systems may provide improvements in the frequency response and dispersion of sound from such horns, none teach or describe a relatively simple and inexpensive mechanism for substantially eliminating "beaming", a phenomena caused by a narrowing of the beam width at high frequencies due to the use of substantially large diameter transducers causing improper diffracting of the acoustic wavefront into the horn. Another problem associated with the use of drivers with relatively large exits which diffract acoustical wavefronts from a circular entrance hole into a substantially large width rectangular slot, is drop-out of the high frequency response along the central axis of the horn due to interference between the wavefronts causing cancellation of high frequency sounds.
The present inventor has recognized that one of the major problems with using a large throat such as a 2-inch-throat format, for example, to obtain high output for high-frequency horns is that beaming typically occurs above 10 kHz in conventional horn designs. Such beaming is a consequence of a physical law that states that as the effective area of the sound source increases in size (the diameter of the throat of a driver, for example), the dispersion angle of the sound source decreases at certain frequencies. Accordingly, relatively small-throat designs, that is, smaller than 2-inch throat designs, for example, have better dispersion characteristics. In conducting various experiments, it was discovered that the vertical polar frequency response for a horn with a 2-inch throat had an undesirable drop-out in the frequency range from 7 to 16 kHz. A horn having a 2-inch throat and flared side portions having an angle of 20 degrees from the horizontal, for example, causes a smaller path-length for sound from the throat to the front of the speaker along the central axis in comparison with the path lengths for sound waves reflected from the sides of the horn along paths parallel to the central axis. The lesser path length for sound from the central axis relative to sound reflected from the sides of the horn creates an interference phenomenon which causes drop-out in the higher frequency ranges, and most particularly near 10.5 kHz. The undesirable drop-out or interference effects are magnified when a circular wavefront from a driver is made to change to a rectangular wavefront in that the middle section of the wavefront which thereby approximates a rectangle is not affected by the transition, but the sound energy at the top and bottom of the circular wavefront is compressed into the central portion of the rectangular wavefront. When, for example, sound wave energy exits from a circular waveguide into a rectangular waveguide, the rectangular portion of the circular wavefront fills the rectangular portion of the horn with some regularity, but the components of energy at the center of the wavefront are driven outward in order to fill the rectangular section evenly, resulting in reflection interference and frequency drop-out due to cancellation of the wavefront at certain frequencies.